As illustrated by FR-A-2 477 265, a preheating steam generator equipping a nuclear power station conventionally comprises a vertically axial, outer envelope, whose inner space is divided into two parts, by a horizontal plate known as a tube plate. The ends of the tubes of a bundle of inverted U tubes are fixed to the tube plate and issue below the latter respectively in an entrance collector and in a discharge or exit collector for the water flowing in the primary circuit of the reactor, known as primary water. The water circulating in the secondary circuit of the reactor, referred to as feed or secondary water, is injected into the part of the steam generator positioned above the tube plate by a semi-torroidal feed collector positioned slightly above an inner envelope covering the tube bundle and having a lower edge spaced from the tube plate. More specifically, the secondary water passes out of the feed collector by inverted J feed tubes, whose lower end issues into a preheating space formed below the feed collector, between the inner envelope and a skirt partly surrounding the latter. This preheating space is located above the primary fluid discharge collector.
As a result of this arrangement, the secondary water introduced in the liquid state into the preheating space is progressively preheated as it descends and before it rises again into the inner envelope along the tubes of the bundle and then vaporizes on contact therewith. The steam thus formed traverses water/steam separating means located in the upper part of the outer envelope before leaving the steam generator. The effect of preheating the secondary water in the preheating space is mainly obtained by the water leaving the water/steam separators and known as recirculation water, which descends again between the outer envelope and the inner envelope, partly through the preheating space.
FR-A-2 644 926 proposes an improvement to such a preheating steam generator, which is directed to preventing the secondary water leaving the inverted J feed tubes from rising again above the feed collector under the effect of eddies, which tend to occur in the upper part of the preheating space. One of the constructional embodiments proposed for preventing the secondary water from re-rising consists of partly blocking or obstructing the top of the preheating space by a horizontal plate fixed to the inner envelope and by extending the inverted J feed tubes, in such a way that they traverse this plate and issue directly below the latter into the preheating space.
Although this solution makes it possible to solve the problem caused by the rising again of the secondary water above the feed collector, its construction leads to a certain number of problems.
Thus, the upper part of steam generators containing the water/steam separators and the secondary water feed collector is manufactured separately from the lower part containing the bundle of tubes, the inner envelope covering the same and the skirt partly surrounding the inner envelope. The two parts of the outer envelope are then welded together. The positioning of extensions for the inverted J feed tubes only takes place after hydraulic testing of the secondary side of the steam generator. For this purpose, manholes are provided in the outer envelope, to enable workers to enter the envelope. However, the fixing of extensions of tubes by welding at this stage of manufacture is difficult to accept, because it may lead to polluting deposits on the tube plate.
Moreover, the two-part manufacture of steam generators, combined with manufacturing tolerances, leads to axial misalignments of different values for each assembly formed by an inverted J feed tube and the corresponding perforation formed in the anti-return plate. The fitting of each of the extensions consequently requires the use of dummies making it possible to take measurements on each of these assemblies, so as to adapt each extension to the assembly corresponding thereto. This leads to a significant time loss, which is difficult to accept at this stage of manufacture, where deadlines are generally very tight.
To these problems encountered during the manufacture of the steam generator are added mechanical behavior and strength problems when the apparatus is operating. Thus, the differential expansions which occur, particularly between the operating periods, of the reactor and the shutdown periods may lead to breaks in the extensions.